title: Recruitment order of quadriceps motor units: femoral nerve vs. direct quadriceps stimulation.
abstract: BACKGROUND
  To investigate potential differences in the recruitment order of motor units (MUs) in the quadriceps femoris when electrical stimulation is applied over the quadriceps belly versus the femoral nerve.
  
  
  METHODS
  M-waves and mechanical twitches were evoked using femoral nerve stimulation and direct quadriceps stimulation of gradually increasing intensity from 20 young, healthy subjects. Recruitment order was investigated by analysing the time-to-peak twitch and the time interval from the stimulus artefact to the M-wave positive peak (M-wave latency) for the vastus medialis (VM) and vastus lateralis (VL) muscles.
  
  
  RESULTS
  During femoral nerve stimulation, time-to-peak twitch and M-wave latency decreased consistently (P < 0.05) with increasing stimulus intensity, whereas, during graded direct quadriceps stimulation, time-to-peak twitch and VL M-wave latency did not show a clear trend (P > 0.05). For the VM muscle, M-wave latency decreased with increasing stimulation level for both femoral nerve and direct quadriceps stimulation, whereas, for the VL muscle, the variation of M-wave latency with stimulus intensity was different for the two stimulation geometries (P < 0.05).
  
  
  CONCLUSIONS
  Femoral nerve stimulation activated MUs according to the size principle, whereas the recruitment order during direct quadriceps stimulation was more complex, depending ultimately on the architecture of the peripheral nerve and its terminal branches below the stimulating electrodes for each muscle. For the VM, MUs were orderly recruited for both stimulation geometries, whereas, for the VL muscle, MUs were orderly recruited for femoral nerve stimulation, but followed no particular order for direct quadriceps stimulation.
authors.affiliation: Department of Electrical and Electronical Engineering, Universidad Pública de Navarra D.I.E.E, Campus de Arrosadía s/n., 31006, Pamplona, Spain, javier.rodriguez.falces@gmail.com.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodriguez-Falces
authors.forename: Nicolas
authors.initials: N
authors.lastname: Place
mesh_terms: D000328:Adult; D005260:Female; D005267:Femoral Nerve / Q000502:physiology*; D006801:Humans; D008297:Male; D052097:Quadriceps Muscle / Q000294:innervation / Q000502:physiology*; D011999:Recruitment, Neurophysiological; D004561:Transcutaneous Electric Nerve Stimulation